1. Technical Field
The invention relates to a turbocharger system including a turbine which is driven for rotation by an exhaust gas from an engine to drive a compressor for controlling a supercharged pressure of an intake air supplied to the engine, and more particularly, to an acceleration detecting device for the turbocharger system which provides an accurate and reliable detection of the acceleration.
2. Related Art
A turbocharger system includes a waste gate valve which controls the bypassing of an exhaust gas supplied to a turbine from an engine. If a supercharged pressure supplied to the engine exceeds a given value, such pressure operates the waste gate valve to open, thereby preventing the supercharged pressure of intake air supplied to the engine from exceeding a given normal value to thereby protect the engine. However, in consideration of the strength limits of various parts used in the engine, a supercharged pressure in excess of a normal value can be tolerated if it persists for a reduced length of time. Accordingly, during an accelerating phase, a solenoid valve which controls the waste gate valve is subject to a duty control to increase the particular point of the supercharged pressure at which the waste gate valve is opened, thereby providing an overboost control in which the supercharged pressure supplied to the engine is temporarily increased beyond its normal value, thus improving the accelerating performance of a vehicle on which the engine is mounted. If the overboost control is allowed to continue for an increased length of time, an excessive load will be applied to the engine, giving rise to the likelihood that the engine may be damaged. Accordingly, the supercharged pressure is returned to the normal value after a given time interval.
The determination of an acceleration is of importance in this process. A technique is proposed, as disclosed in Japanese Publication of Unexamined Patent Application No. 180,726/1983, for example, in which the determination is based on the position of a throttle valve, by seeing if it passes through a given opening which is very close to a full opening. However, this technique can only be used when the acceleration takes place under nearly fully open condition, and the determination is difficult to achieve for a partial acceleration. Thus, the overboost control in such region is disabled, resulting in an accelerating performance which is less than desired.
As an alternative, the acceleration may be determined by using a rate of change in the supercharged pressure in the increasing direction. However, when the supercharged pressure is detected by a sensor which is directly mounted on the engine, noises from the engine or the condition of combustion within the engine cause the measured supercharged pressure output to change rapidly up and down rather than providing a smooth curve. Hence it is difficult to obtain a reliable rate of change in the supercharged pressure in the increasing direction within a reduced length of time.
Another problem relates to the retarded determination of the acceleration when the supercharged pressure increases rapidly. Thus, the maximum supercharged pressure may be reached before the overboost control is initiated. Consequently, there may be a time period when the overboost control is not effective during the early stage which is significant in the accelerating process.